Determining the concentration of orthophosphate PO43− in process analysis, especially in the application domains of water and waste water, is of worldwide interest. This parameter plays an important role especially for monitoring bodies of water, for monitoring the maintaining of prescribed P discharge limit values in industrial and community clarification plants, and, in given cases, for the control of the phosphate elimination, respectively phosphate reclamation, performed in such clarification plants.
To this point in time, the determining of orthophosphate in these applications is frequently performed by means of laboratory analyses. As a rule, this permits no near in time determining of measured values. A providing of measured values in real time, such as is required for the automated control of processes, is likewise not possible by means of laboratory analyses.
To this point in time, known automatic analysis measuring devices for orthophosphate determination, especially on-line measuring devices, can, indeed, in principle, be applied for automated control. However, the measured value determination in these analytical measuring devices is based on wet chemical analytical methods. These require the application of chemical reagents, with which a liquid sample is treated, in order to bring about an, as a rule, photometrically detectable change of the properties of the liquid sample, e.g. a coloring or a color change, dependent on the orthophosphate content. These analytical devices deliver, indeed, very exact measurement results. However, they are relatively complicated in construction and require significant maintenance. Especially, reagents and, in given cases, wear parts need regularly to be replaced.
In situ sensors, such as optrodes or ion-selective electrodes, form for determining concentration of many ions a relatively cost effective and less maintenance demanding alternative, which virtually continuously provide measured values of ion concentrations without sample taking and without addition of reagents. An ion-selective electrode is a potentiometric sensor having a measuring half cell and a reference half-cell. The measuring half cell includes a measuring membrane, at which a potential arises dependent on the concentration of a certain kind of ion. Used as reference half-cell can be, for example, a reference electrode of second type, e.g. a silver/silver chloride, reference electrode, which provides a stable reference potential independent of the measured variable. The determining of ion concentration occurs based on the potential difference registered between the measuring half cell and the reference half-cell when in contact with a measured liquid. A number of examples of ion-selective electrodes are given, for example, in K. Cammann, H. Galster, Das Arbeiten mit ionenselektiven Elektroden (Working with Ion-Selective Electrodes), 3rd Edition, Springer Verlag (publisher), 1996.